Seals for utilization in appliances (such as, refrigerators, freezers, and ice makers), automotive body applications (such as, windows, hoods, trunks and doors), and building applications (such as window glazing gaskets and weather strips) should be dimensional stable, provide low compression set and offer outstanding sealing characteristics over a broad temperature range. In many applications it is important for the seals to be capable of sealing out noise, wind and water, while providing long-term ultraviolet light resistance. At the same time, the material used in the seal must offer the degree of flexibility required for the particular application. Window and door weather stripping for automobiles and trucks is a high volume application for such seals. However, seals offering essentially the same characteristics are also needed for sun roof seals, handle gaskets, window spacers, window guides, lock seals, windshield wiper pivot seals and in building applications such as window glazing gaskets and weather seals. Gaskets utilized in plumbing applications and appliances must also be capable of forming a liquid/gas tight seal and low compression set.
Rubbery blends of polyvinyl chloride (PVC) with a nitrile rubber (NBR) have sometimes been used in seals for automotive body applications. The nitrile rubber is included in such blends as a permanent modifier for the PVC which provides it with a higher degree of flexibility. However, the utilization of standard nitrile rubber in such blends typically results in only moderate compression set characteristics. It is very important for seals to have good compression set characteristics in most applications. For instance, improved resistance to water leaks and wind noise can be attained by utilizing a seal which has low compression set characteristics.
It is known from the teachings of United Kingdom Patent Application No. 9214969.9 that low compression set characteristics can be improved by utilizing a technique known as “dynamic vulcanization” via free radical generators, such as azo compounds or organic peroxides. However, this “dynamic vulcanization” technique suffers from the weakness that the azo compounds or organic peroxides required reduce the thermal stability of the polyvinylchloride resin and the ultraviolet light resistance of the nitrile rubber. There is also an increased risk of early crosslinking during processing which leads to scorching and reduced recyclability.
U.S. Pat. No. 5,362,787 discloses a highly crosslinked nitrile rubber which can be easily blended with PVC to make compositions which have an excellent combination of properties for use in making seals and gaskets for automotive and building applications. The PVC blends made with such highly crosslinked nitrile rubbers offer excellent dispersion behavior, dimensional stability, low compression set, outstanding sealing characteristics, and low temperature flexibility.
U.S. Pat. No. 5,362,787 more specifically discloses a highly crosslinked nitrile rubber composition which can be blended with polyvinyl chloride to make compositions having excellent characteristics for seals and gaskets including dimensional stability, low compression set, outstanding sealing characteristics and good low temperature flexibility, said highly crosslinked nitrile rubber composition being comprised of (1) a highly crosslinked nitrile rubber having repeat units which are derived from (a) 1,3-butadiene, (b) acrylonitrile and (c) a crosslinking agent, wherein said highly crosslinked nitrile rubber has a Mooney viscosity of about 50 to about 120, a swelling index of less than about 10 percent, a mill shrinkage of less than 10 percent, and a gel content of greater than 90 percent; and (2) from about 1 to about 30 phr of a plasticizer.
U.S. Pat. Nos. 5,380,785, 5,415,940 and 5,462,993 disclose a rubbery polymer which can be blended with polyvinyl chloride to make leathery compositions having good heat and ultraviolet light resistance, said rubbery polymer being comprised of repeat units which are comprised of (a) butyl acrylate, or optionally a mixture of butyl acrylate and 2-ethylhexyl acrylate containing up to about 40 percent 2-ethylhexyl acrylate, (b) at least one member selected from the group consisting of methyl methacrylate, ethyl methacrylate, methyl acrylate and ethyl acrylate, (c) acrylonitrile, (d) styrene, (e) a half ester maleate soap and (f) a crosslinking agent.
U.S. Pat. No. 5,927,029 discloses a polymeric composition having excellent characteristics for seals and gaskets including dimensional stability, low compression set and outstanding sealing characteristics, said polymeric composition being comprised of a vulcanized blend of (1) a rubbery polymer which is comprised of repeat units which are derived from (a) butyl acrylate, (b) at least one member selected from the group consisting of methyl methacrylate, ethyl methacrylate, methyl acrylate, and ethyl acrylate, (c) acrylonitrile, (d) styrene, and (e) a crosslinking agent; (2) a thermoplastic styrene/ethylene-butylene/styrene resin; and (3) at least one glycol component selected from the group consisting of ethylene glycol and triethylene glycol.